R. Street

Near‐surface deformation in the New Madrid Seismic zone as imaged by high resolution SH‐wave seismic methods

Over 1.5 kilometers of high-resolution, SH-wave refraction and reflection profiles have been performed in the Madrid Bend area of northwestern Tennessee, approximately 8 km north of Tiptonville, Tennessee, to establish the existence of neo-tectonic, near-surface deformation. The specific area was chosen because of the access it provided to the Tiptonville dome (a Holocene flexure) and the trend of contemporary seismicity within the New Madrid seismic zone (NMSZ). We believed the highest likelihood of near-surface deformation to be over these features. Partially reversed refraction/reflection walk-away tests and common depth point (CDP) seismic profiling, using the non-traditional SH-wave method, have traced antiformal flexure and numerous faults into the late Eocene and Holocene sediments of the Tiptonville dome province of the Lake County uplift (LCU). Vertical displacements exhibited across the faults range from approximately 3 to 30 m. These structures have been interpreted to be neo-tectonic features associated with tensional stresses across the Tiptonville dome.

Soil classifications for estimating site-dependent response spectra and seismic coefficients for building code provisions in western Kentucky

Abstract One hundred thirty-four horizontally polarized shear-wave (SH) seismic soundings have been performed in the Jackson Purchase Region of western Kentucky. These data have been used to derive the thicknesses and shear-wave velocities for the various impedance horizons in the upper 30 m of the soil column at each site for the purposes of general land-use management and loss estimation in the event of an earthquake. A generalized seismic susceptibility soil classification scheme adopted by the Borcherdt (1994a,b) was fit to the results. Soils across western Kentucky were found to be either SC-II or SC-III soils, correlating to gravelly soils having soft to firm consistency and stiff clays to loose/dense sands, respectively.

Site effects at a vertical accelerometer array near Paducah, Kentucky

Abstract The downhole vertical accelerometer array VSAP near Paducah, KY, consists of three-component accelerometers at the surface, the top of the McNairy Formation (−41 m), and the top of the Paleozoic bedrock (—99 m). The array is at the northern end of the Mississippi Embayment, and it was installed to verify the ground-motion modeling for the site, assuming a significant earthquake in the New Madrid Seismic Zone. Accelerograms from 4.2 and 2.0 mb earthquakes were used to check aspects of the modeling pertaining to linear behavior of the soil column, and to review the soil column models derived by drilling and geotechnical methods and through the use of high-resolution P- and SH-wave seismic refraction in reflection techniques. Results of the study indicate that for the linear case the soil column models derived by the two techniques are equivalent, and that the most important boundary in the soil column, with respect to amplification of the ground motions, is the interface between the limestone bedrock and soil.

Shallow seismic reflection investigations of neotectonic activity in the Lower Mississippi Valley

Summary Shallow shear wave seismic reflection methods were used to help interpret the significance of neotectonic surface deformation at three sites in the Lower Mississippi Valley. The seismic data were acquired with a hammer and mass energy source and processed using a standard sequence for shallow reflection data. The interpreted profiles show a range of shallow structural styles that includes reverse faulting, fault propagation folding, and reactivated normal faulting. Determining the style and extent of near-surface structural deformation is critical in evaluating the seismic potential of an area.

Seismic Evaluation of the US41 Northbound Bridge over the Ohio River at Henderson, KY

The main objective of this investigation is to assess the structural integrity of the Ohio River bridge on US 41 Northbound at Henderson, Kentucky, when subjected to a projected 50-year earthquake. The investigation considers both the main bridge and the approach spans. To achieve the objective, the scope of the work was divided into the following tasks: (1) Field testing of the main bridge; (2) Finite element modeling and calibration; (3) Time-history seismic response analysis; and (4) Seismic response of the approach bridges using the response spectrum method.

Seismic investigation of near-surface geological structure in the Paducah, Kentucky, area: application to earthquake hazard evaluation

Abstract Seismics method were used to evaluate shallow geological conditions at 33 sites in the vicinity of Paducah, Kentucky. A combined set of P- and S-wave seismic refraction and reflection soundings were used, in addition to local borehole information, to produce structure maps of (1) a shallow ( 160 m deep). Shear-wave velocity contrasts across the shallow unconformity were generally 2-to-1 while the contrast at the top of the Paleozoic bedrock exceeds 5-to-1. These seismic boundaries have been determined to be very important in modelling and interpreting earthquake ground motion amplification in the Paducah area. The quality and accuracy of the data, and the cost effective nature of the methods, suggest that other communities in areas at risk to damage from seismic activity, with foundation conditions comparable to Paducah, might benefit from similar characterization in order (1) to identify seismically hazardous, near-surface, geological conditions, and (2) to develop geological models that could be used in computer simulations of site response.

SH-wave refraction/reflection and site characterization

Traditionally, nonintrusive techniques used to characterize soils have been based on P-wave refraction/reflection methods. However, near-surface unconsolidated soils are often times water-saturated, and when groundwater is present at a site, the velocity of the P-waves is more related to the compressibility of the pore water than to the matrix of the unconsolidated soils. Conversely, SH-waves are directly relatable to the soil matrix. This makes SH-wave refraction/reflection methods effective in site characterizations where groundwater is present. SH-wave methods have been used extensively in site characterization and subsurface imaging for earthquake hazard assessments in the central United States and western Oregon. Comparison of SH-wave investigations with geotechnical investigations shows that SH-wave refraction/reflection techniques are viable and cost-effective for engineering site characterization.

SEISMIC EvALUATION OF THE OHIO RIVER BRIDGE oN US51 AT WICKLIFFE, KENTUCKY

This report presents the use of seismic evaluation of the Ohio river bridge on US51 at Wickliffe, Kentucky. The main bridge is a five-span single-deck cantilever through-truss type. The approach bridge has 21 spans on the Kentucky side and 6 span single-deck cantilever through-truss type. The approach bridge has 21 spans on the Kentucky side and 6 spans on the Illinois side. Although this bridge has not yet been subjected to a moderate or major earthquake, it is situated within the influence of the New Madrid seismic zone. The seismic evaluation program consists of field testing and seismic response analysis. The modal properties of the main bridge are determined through field testing, and are used to calibrate the three dimensional finite element model. The finite element model is then subjected to time histories of the 50-year earthquake event. Stresses and displacements obtained are within the acceptable limits. Analytical results indicate that the main bridge will survive the projected 50-year earthquake without significant damage and no loss-of-span. Hence, it is not recommended to retrofit the main bridge. The approach spans are analyzed using response spectrum method with simplified single-degree-of-freedom models. Most of the Kentucky and Illinois approach spans require additional anchor bolts at the bearings.

A Note on Blast‐Monitor and Conventional Seismic Network Recordings of Moderate‐Sized Earthquakes in the Central United States

Abstract Since the mid 1980s small and moderate‐sized earthquakes in the Ohio and Wabash River valleys of the central United States (CUS) have been digitally recorded by seismographs, called blast monitors, deployed to detect and characterize vibrations from explosions associated with regional mining and quarrying. Because there were relatively few conventional networked strong‐motion and broadband instruments for this area between 1980 and the early 2000s, the more than 200 collected observations have provided a widespread source of digital earthquake ground motions. Additional deployment of networked instrumentation during the last decade and their numerous recordings of the 18 April 2008 Mt. Carmel Illinois earthquake sequence have provided the first effective means for comparing free‐field blast‐monitor and conventional network ground motion observations. The peak ground motion characteristics for both data sets relative to a common predictive relationship are similar, suggesting that blast monitor observations compliment conventional network data for small and moderate‐sized ( M

DYNAMIC SITE PERIODS FOR THE JACKSON PURCHASE REGION OF WESTERN KENTUCKY

For this report, conventional seismic refraction and reflection techniques were used to determine the shear-wave velocities of the more poorly consolidated, near-surface sediments for a matrix of sites in the region. Conventional seismic P-wave reflections along with existing drill hole and seismic reflection data in the region were then used to determine the depth to the top of the bedrock at the sites investigated. These data were used in SHAKE91 to calculate the fundamental period of the ground motion at the sites. This period, identified in the study as the dynamic site period, is the period at which ground motions in the sedimentary column are most apt to be amplified as a result of a seismic shear wave propagating from the top of the bedrock to the surface. Based on the results in this report, it is recommended that bridges, overpasses, and other engineered structures built in the region be designed so that their natural periods do not coincide with the fundamental period of the sedimentary column, thereby avoiding damage during an earthquake as a result of in-place resonance.